The LHeC aims at the generation of hadron-lepton collisions with center of mass energies in the TeV scale and luminosities of the order of $10^{32}-10^{33} cm^{-2} sec^{-1}$ by taking advantage of the existing LHC 7 TeV proton ring and adding a high energy electron accelerator. This paper presents technical considerations and potential parameter choices for such a machine and outlines some of the challenges arising when an electron storage ring based option, constructed within the existing infrastructure of the LHC, is chosen

Bordry, Frederick

Braun, H H

Brüning, Oliver Sim

Burkhardt, H

Chattopadhyay, S

Dainton, J B

Holzer, B

Jowett, John M

Klein, M

Linnecar, T

Mess, K H

Myers, S

Osborne, J

Willeke, F

Zimmermann, F

CERN Document Server

CERN-AB-2008-04104Sep2008A STORAGE RING BASED OPTION FOR THE LHECThe LHeC aims at the generation of hadron-lepton collisions with center of mass energies in the TeV scaleand luminosities of the order of 1032–1033 cm-2 sec-1 by taking advantage of the existing LHC 7 TeV protonring and adding a high energy electron accelerator. This paper presents technical considerations andpotential parameter choices for such a machine and outlines some of the challenges arising when anelectron storage ring based option, constructed within the existing infrastructure of the LHC, is chosen.EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN   -   AB DepartmentGeneva, SwitzerlandAugust 2008AbstractCERN-AB-2008-0411) BNL, Upton2) Cockcroft Institute, Daresbury3) DESY, Hamburg4) Liverpool UniversityF. Willeke1), F. Bordry, H. H. Braun, O. Brüning, H. Burkhardt, S. Chattopadhyay2), J.B. Dainton2),B. Holzer3), J.M. Jowett, M. Klein4), T. Linnecar, K.H. Mess, S. Myers, J. Osborne, F. ZimmermannCERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008A STORAGE RING BASED OPTION FOR THE LHECF. Willeke, BNL, Upton, New York, USAF. Bordry, H. Braun, O. Bru¨ning, H. Burkhardt, J.M. Jowett, T. Linnecar,K.H. Meß, S. Myers, J. Osborne, F. Zimmermann, CERN, Geneva, Switzerland,S. Chattopadhyay, J.B. Dainton, Cockcroft Institute, Daresbury, UK,B. Holzer, DESY, Hamburg, Germany, M. Klein, Liverpool University, UKAbstractThe LHeC aims at the generation of hadron-lepton col-lisions with center of mass energies in the TeV scale andluminosities of the order of 1032–1033 cm−2 sec−1 by tak-ing advantage of the existing LHC 7 TeV proton ring andadding a high energy electron accelerator. This paperpresents technical considerations and potential parameterchoices for such a machine and outlines some of the chal-lenges arising when an electron storage ring based option,constructed within the existing infrastructure of the LHC,is chosen.INTRODUCTIONIt was originally foreseen to allow for both an electronring (LEP itself in the earliest versions) and a hadron ringin the LHC tunnel [1]. Interest in a lepton-hadron col-lider, LHeC, was rekindled recently by the proposal to adda new lepton ring to the LHC [2]. Here we build on thatstudy to look more closely into aspects of the lepton ring.This could store positrons or electrons given investment insources and polarity-switching capability. An alternativering-linac option is discussed in [3] and more general as-pects in [4].The main parameters, from [2], are summarized in Ta-ble 1 (see also [5]) and are driven by assumptions on RFpower available. Clearly there is ample scope for staginginstalled power and e± energy from 50–70 GeV, ultimatelyto approach the parameters of Table 1. If, e.g., the lep-ton beam current is kept constant, the e± energy scales asE ∝ P 1/4RF.Table 1: Main parameters for e±p collisionsQuantity unit e± pBeam energy GeV 70 7000Total beam current mA 74 544Particles/bunch Nb 1010 1.40 17.0Horiz. emittance nm 7.6 0.501Vert. emittance nm 3.8 0.501Horizontal β∗x cm 12.7 180Vertical β∗y cm 7.1 50Energy loss per turn GeV 0.707 6× 10−6Radiated power MW 50 0.003Bunch frequency MHz 40CMS Energy (√s) GeV 1400Luminosity /1033 cm−2s−1 1.1LAYOUT AND BYPASSThe idea is to add a lepton ring to the LHC with mini-mal interference for the continuing high-luminosity pp pro-gram. This will require a separation bypass for the leptonring around the high luminosity experiments ATLAS, inthe interaction region IR1, and CMS in IR5. We assumethat the low-luminosity LHC insertions, IR2 and IR8, canbe adapted to the needs of the LHeC with RF, injectionand new experiments. Fig. 1 shows the LHC undergroundstructures and extensions considered for the LHeC whileTable 2 lists the bypass tunnels considered necessary. RH2 3RH 87UJ 4 6UA4 7UJ 4 7RA4 7UW 4 5US 4 5UL4 6TX4 6UJ 4 4UX4 5RA4 3UA4 3UL4 4UJ 4 3 RR 53UJ 5 3UXC5 5UL 54US C5 5P M5 4P X5 6RZ5 4UP 5 3UJ 5 6 1UJ 5 7RR 57UJ 5 6P M5 6UL5 6TU5 6UD 62UJ 62UJ 63P M6 5UJ 6 4UA6 3RA6 3TD 62UP 62UL6 4P Z6 5P X6 4UJ 6 6UJ 6 7UJ 6 8UX6 5UA6 7RA6 7TD6 8UD6 8UP 68UL6 6TX6 4UW 6 5 US 6 5Point 7RR7 3RR7 7UJ 7 6P M7 6TZ7 6RA8 3UA 83UJ 83UJ 8 4UJ 8 2Point 8 PM 85P X8 4P Z8 5UX 85TX8 4UL 84UA8 7RA 87UJ 86UJ 8 7UW 8 5US 85UL 86TI 8 UJ 88PGC 8TJ  8  RR1 3UJ 1 3RT1 2UJ 1 4US 1 5TI 1 2P M1 5P X1 4UX1 5UL1 4UJ 12RE 12 RE 88LS S 4P o in t  1 .8PMI 2UJ 1 7UJ 1 8UJ 1 6TI 1 8RR1 7PM 18 P X1 6 P X1 5US A1 5UL1 6UJ 2 2UJ 23UJ 2 4UA2 3RA2 3TI  2P GC2RA2 7UJ 2 6P X2 4UX2 5P M2 5UW 2 5US 2 5UL 26UL 24UJ 2 7 UA2 7 Point 2 Point 4P Z 3 3P M3 2UJ 3 2 UJ 3 3RZ3 3TZ3 2Point 5  Point 6 Point 1  S P S  P X4 6P Z4 5P M4 5RT 18P o in t  3 .3P o in t  3 .2UP 2 5TT 40LEPLHCRE 32RE 28RE 38RE 42RE 48RE 52RE 58RE 62RE 68RE 72RE 78RE 82RE 22RE 18NLHCBATLASALICECMSLHeCe-injectorepepRFRFRFRFFigure 1: Civil engineering for the LHeC.The RF system will require shielded areas to housethe RF power amplifiers and associated electronics. Atlower energies, 50 GeV, say, the superconducting cavities( 1GHz) can be accommodated at one long straight section(LSS). At higher energy it may be preferable to divide thesystem between two LSSs to avoid beam dynamics effectsassociated with high synchrotron tune and energy. The totalnumber of cavities will depend on the RF coupler capabilityrather than the accelerating gradients (MV/m) achievable.Table 2: Bypass tunnels and approximate dimensions.IR1/IR5 IR2 and/ IR3/IR7ATLAS/CMS or IR 8Bypass for Experiments RF CollimationDiameter 4.4/3.8 m 5.50 m 4.2/3.8 mLength 500 m 500 m 500 mSeparation 10–13 mQL18 QL1810 m 237.034 mQL11 QL1122 maccelerator tunnelcavern-10-5 0 5 10-600 -400 -200  0  200  400  600LEPBypassx [m]y [m]Figure 2: By-pass layout study, derived from the LEP lat-tice without addition of bending magnets. The y-scale isstretched by a factor of 50.INTERACTION REGION (IR)The bypasses around the experiments in IR1 and IR5 areexpected to be the most demanding because they require10–13 m of horizontal separation at the interaction point(IP). We have therefore started to look into possible designsin some detail. Fig. 2 shows a bypass which was derivedfrom the LEP lattice without addition of bending magnets,thus avoiding any increase in synchrotron radiation losses.The design of the new interaction region for the e±-hadron collisions is based on the principle that the twohadron storage rings of the LHC can be operated indepen-dently of the lepton ring. This is an unavoidable conse-quence of the need to maintain the standard pp operation ofthe LHC simultaneously with e±p collisions and the factthat the lepton and hadron beam rigidities differ by a fac-tor of a hundred. The principle of the IR layout has beenpresented in various papers and a possible arrangement ofthe low-β quadrupole lenses, as discussed, e.g., in [2], isshown in Fig. 3.While the 7 TeV proton beam will pass through the mini-β triplet of the lepton storage ring, any proton magnet willonly be placed where the two beams are sufficiently sep-arated so the electron beam is not affected by the muchstronger magnetic fields of the proton lenses. The focus-ing and the separation are therefore closely coupled. Onthe other hand, whatever the design of the IR, the protonswill be affected by some focusing and bending fields of theelectron ring. Therefore the IR layout presented here isbased on a robust proton beam optics that is adequate forthe required luminosity and can also tolerate (and compen-sate) the perturbation by the common magnets as they varythrough the full operational cycle of the electron storagering. Thanks to the large difference in momentum it can beFigure 3: Schematic interaction region layout.LHeC optics triplet√β  [√m]Dispersion x / z  [m]s [m]Figure 4: Hadron beam optics for e-p collisions in IP8assumed that the fields of the lepton ring have little impacton the hadron beam and compensation of the optics and theorbit will not be difficult.Furthermore, the large momentum difference of the twocolliding beams provides a very elegant way to separatethe lepton and the hadron beam: Shifting the mini-βquadrupoles of the electron beam and installing a long butweak bending magnet close to the IP provides the gen-tle separation scheme needed to keep the synchrotron ra-diation level in the IR within reasonable limits. For thehadron beam optics presented here, the constraints arisefrom the β-functions at the IP (Table 1), the separationscheme (mainly the length of the dipole separator mag-net determining the distance of the first hadron quadru-pole to the IP) and, above all, matching to the present LHCFODO structure in the arc. The basic problem for any e±pcollision scheme is that the present layout of IR8 is anti-symmetric in that the first main superconducting quadru-pole on the right of the IR is focusing, on the left, defo-cusing. While anti-symmetric solutions for a round beamoptics (βx = βy) are easily established, flat e± beams(ǫx ≃ 2ǫy) require unequal β-functions for the hadronbeam and a symmetric quadrupole arrangement can nolonger be maintained. Thus, the hadron beam optics pro-posed here is based on a mini-β triplet on both sides of theIP in which the focusing magnets have different strengthson the right and left of the IR.Figure 4 shows a possible solution. The first quadrupoleis located at a distance of 22 m from the IP, determined bythe separation scheme of the lepton beam. The β-functioninside the mini-β quadrupoles reaches a maximum valuein both planes of about 2200 m, low enough to provideflexibility and aperture for beam separation during leptoninjection and to allow the lepton quadrupole magnets tobe powered for either e± running mode. The influence ofthe lepton magnets on the hadrons will be compensated lo-cally, i.e., correction currents will be applied in the hadronlow β quadrupoles and orbit corrector dipoles to counteractthis external distortion as a function of the e± energy andcharge.Special care has to be taken at lower hadron energiesduring injection and acceleration as the emittance of thehadron beam scales inversely with energy and will be aslarge as 8µm at injection. It is not expected that a lumi-nosity optics as presented in Fig. 4 will be required. Insteadone must find an optical solution where the emphasis is puton aperture rather than minimum β at the IP.A first result of such a beam optics is sketched in Fig. 5:based on the same element layout as in Fig. 4, it has relaxedaperture requirements. Work is in progress to find a smoothinterpolation between the two extreme situations.LHeC optics triplet√β  [√m]Dispersion x / z  [m]s [m]Figure 5: Hadron beam optics for injection and ramp e-pIn practice, the operational cycle of the LHeC IR in thecase of an e±p run will be as follows: first, the proton beamwill be injected into an optics like Fig. 5 which is thenramped up to top energy of 7 TeV. During this procedurethe lepton ring magnets, (triplet quadrupoles and separatordipole) are at minimum fields to minimise perturbations ofthe more sensitive 450 GeV proton beam. At top energy inthe hadron ring, separation bumps will be set up at the IPto prepare for the lepton injection.Then the lepton magnets will be cycled to their injectionsettings and leptons will be injected and ramped to their topenergy. During these procedures, correction settings in theproton ring will continuously compensate for the perturba-tion from the lepton ring. It is foreseen to keep the opticsof the lepton ring constant through injection, ramp and col-lisions, so that the compensation currents for the protonswill be scalable throughout. Finally, having accelerated thelepton beam, the β-squeeze optics of the protons will be ap-plied, and the separation bump switched off, to bring theminto collision with the leptons.LEPTON-NUCLEUS COLLISIONSIt is natural to take advantage of the LHC’s second roleas a nucleus-nucleus collider to extend its physics reachwith e±-ion collisions. Here we briefly consider the po-tential of using the existing ion injector chain to providee±-Pb collisions, keeping in mind that a range of both lightand heavy nuclei are expected to be available in the LHCin coming years.Let us assume the same number (592), intensity (7×107ions/bunch) and emittance of 208Pb82+ ion bunches as forthe present nominal Pb-Pb collision parameters [5] andthat the lepton injectors can create matching trains of lep-ton bunches. The Pb beam sizes are the same as the pro-tons’. With the same single bunch current as for e±p (Ta-ble 1), we find a lepton-nucleus luminosity L ≈ 1.09 ×1029cm−2s−1 (or a lepton-nucleon luminosity of Len ≈2.2 × 1031cm−2s−1) but with the radiated power reducedby the ratio of the number of bunches, 2800/592 ≈ 4.7.In principle, the lepton bunch current (hence, the luminos-ity) could be increased by about this factor to exploit theRF power available for e±p operation without exceedingsingle-bunch current limits.LEPTON INJECTORSThe LEP pre-injectors have been dismantled and the in-frastructure re-used for the CLIC test facility CTF3. TheRF cavities that accelerated leptons in the SPS have beenremoved to reduce its impedance. Re-installation of an in-jector chain similar to LEP’s through the PS and SPS wouldbe costly and potentially limit the proton performance sothe LHeC needs new lepton injectors [6]. The requiredbunch intensities of 1.4× 1010 are well below the 4× 1011used for LEP. This should lower the injection energy fromthe 22 GeV of LEP and reduce the cost of new lepton in-jectors. A scaled-down version of ELFE [7] may be a can-didate for the LHeC lepton injector.REFERENCES[1] E. Keil, “LHC ep option”, LHC-Project-Report-093, CERNGeneva 1997[2] J. B. Dainton, M. Klein, P. Newman, E. Perez, andF. Willeke, “Deep inelastic electron nucleon scattering at theLHC”, JINST 1 (2006) P10001[3] F. Zimmermann et al., “Linac-LHC Electron-Proton ColliderOptions”, these proceedings.[4] J. Dainton et al., “Prospects for a Large Hadron ElectronCollider (LHeC) at the LHC”, these proceedings.[5] O. Bru¨ning et al. (Eds.), “LHC Design Report Vol. I, TheLHC Main Ring”, CERN-2004-003 (2004).[6] H. Burkhardt, “LHeC Ring-Ring”, DIS2008, Proceedings inpreparation[7] H. Burkhardt (Ed.), “ELFE at CERN.” CERN 99-10.

A Storage Ring based Option for the LHeC

A Storage Ring based Option for the LHeC

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